1. Field of the Invention
The present invention relates to tracking systems. More specifically, the present invention relates to methods and apparatus for locating and tracking communication transponders using phase interferometer techniques.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Automated toll collection systems are known in the art. Typically, these systems comprise a reader transponder and at least one vehicle transponder. The reader transponder includes either laneside, overhead or in-pavement antennas in each traffic lane. The reader transponder senses the presence of the vehicle transponder by employing a vehicle sensor or by transmitting an RF signal to which the vehicle transponder responds with an identification message. In conventional automated toll collection systems, toll booth barriers are utilized to route vehicles one at a time through an electronic toll collection area. Thus, only one vehicle transponder is within the range of the antenna pattern. The toll is then collected electronically from the vehicle in the designated area.
Unfortunately, existing electronic toll collection and tracking systems do not identify and locate all vehicles on a roadway unless the vehicle includes a transponder. If a particular vehicle is not equipped with a transponder, it will not be detected by the reader transponder. Thus, if tolls are only collected from vehicles via an electronic link, the absence of a transponder enables a vehicle to utilize the roadway without charge. Therefore, enforcement of toll collection from vehicles using the roadway is a major problem in electronic toll collection systems of the prior art.
In the past, the problem of enforcement of toll collection from vehicles using the roadway has been addressed in the following manner. A communication link is established between the reader transponder and a particular vehicle transponder by accurately adjusting the antenna pattern of the reader transponder to illuminate and to electronically collect the toll from a single vehicle transponder. These requirements necessitate an antenna to be positioned in each traffic lane of the roadway. Each of the antennas are either connected to a dedicated roadside reader transponder or are multiplexed to a single reader transponder. The systems of the prior art are designed to communicate with only a single vehicle per traffic lane and then only if the lane is equipped with an antenna. Each vehicle must slow down or stop, or remain in a particular traffic lane during the vehicle identification process. These requirements restrict the flow of traffic and tend to increase congestion.
A rudimentary method of enforcement of toll collection from vehicles using the roadway is to design the transponder fitted on each vehicle to be physically observable. A vehicle using the roadway without an observable transponder is cited as a violator. In more sophisticated electronic toll collection system designs, the antenna patterns are used to determine if a particular vehicle is a toll violator. The absence of a transponder response from a vehicle in combination with detection of the vehicle at an established detection location (e.g., for example, video detection) indicates a potential toll violation.
The enforcement performance of the toll collection technique utilizing the antenna patterns is further determined by the ability to electrically isolate the responses of different vehicle transponders from one another. The installation of physical barriers and associated infrastructure to electrically separate vehicles is required to achieve high performance enforcement during the toll collection procedure. Therefore, a conventional electronic toll collection system necessitates one of the following constructions: either (a) a reader transponder antenna must be buried in each lane of the roadway and the vehicle transponders must be mounted on the bumpers of the individual vehicles, or (b) wide lane spacing must be provided for increasing the spacing between vehicles to ensure that overhead antenna patterns can provide the vehicle response separation without ambiguity. Both of these construction alternatives also lead to restrictions on lane changes during the toll collection procedure.
The ability to electrically isolate responses of different vehicle transponders from one another to improve enforcement performance is inversely related to the requirement of ensuring reliable communications between the reader transponder and the vehicle transponder. To improve the reliability of a communication link with a vehicle transponder and thus to increase the probability of toll collection, the antenna radiating power of the reader transponder can be increased. Increasing the antenna radiating power extends the area of coverage of the reader transponder.
Although the reliability of the communication link might be improved, increasing the antenna radiating power of the reader transponder can result in a cross-lane read problem in a high density traffic environment. A cross-lane read problem is characterized by the inability of the reader transponder to determine with which vehicle transponder the communication link has been established. Thus, the vehicle transponder with which the reader transponder is communicating can be erroneously identified. In order to minimize the cross-lane read problem and to isolate vehicle transponder responses from one another, it is necessary to reduce the antenna radiating power of the reader transponder. Unfortunately, this action also reduces the reliability of the communication link between the reader transponder and the vehicle transponder and lowers the toll collection performance level of the electronic toll collection system.
Unfortunately, the above described automated toll collection systems of the prior art do not identify a particular vehicle with respect to the position or location of the vehicle transponder on the roadway. If tolls are electronically collected from vehicles, the absence of a transponder enables a vehicle to utilize the roadway without charge. Clearly, toll collection enforcement of the exemplary automated toll collection systems exhibit poor performance. Thus, the probability of violator identification is low, the infrastructure costs are high since many additional components are required to support the system, and the restrictions on traffic flow are burdensome. Further, the reliability of the system is reduced during high traffic density periods or when vehicles straddle a traffic lane.
Thus, there is a need in the art for improvements in systems that locate and track vehicle transponders associated with automated toll collection systems.